1. Field of the Invention
The present invention relates to a pretensioner for a webbing retractor, which eliminates slackness in a webbing by forcefully winding the webbing onto a winding shaft when a vehicle is in a state of emergency such as a collision.
2. Description of the Related Art
There are webbing retractors which include a pretensioner for eliminating slackness in a webbing by forcefully winding the webbing onto a winding shaft when a vehicle is in a state of emergency such as a collision.
Among such pretensioners, there is what is known as a rack-and-pinion type of pretensioner. In a webbing retractor 100 such as shown in FIG. 9 which is provided with the rack-and-pinion type pretensioner, a pinion 104 is connected to a winding shaft 108 onto which a webbing 106 is wound, such that the pinion always rotates integrally with the winding shaft 108. A cylinder 102 is provided near the pinion 104 so that gas is supplied into the cylinder 102 when the vehicle is in a state of emergency.
A piston 110 is movably provided inside the cylinder 102. When the vehicle is in a state of emergency, the piston 110 is moved in the direction of the arrow B in FIG. 9 by the gas pressure. A rack 112 is provided integrally with the top surface of the piston 110. Due to the rack 112 moving together with the piston 110, the rack 112 meshes with the pinion 104 and rotates the pinion 104 in the direction in which the webbing 106 is wound (the direction of the arrow A in FIG. 9). Accordingly, when the vehicle is in a state of emergency, the webbing 106 is wound onto the winding shaft 108 and the slackness in the webbing 106 is eliminated.
A seal holding groove 114 is formed in the side surface of the piston 110 by a cutting process, and a rubber O ring 116 is fitted into the seal holding groove 114. The O ring 116 seals the gap between the piston 110 and an inner wall 118 of the cylinder 102, thus improving the air-tightness between the piston 110 and the cylinder 102. Further, a vent hole 120 is formed from the bottom surface of the piston 110 through to the top surface thereof. Accordingly, when the pretensioner is operated, the gas passes from the bottom surface side of the piston 110 to and out of the top surface side thereof through the vent hole 120. This prevents the gas pressure inside the cylinder 102 from excessively increasing.
However, in this type of webbing retractor 100, much work is required by the machining (working) for forming the seal holding groove 114 and the vent hole 120 in the piston 110. As a result, costs are increased.
Further, the diameter of the O ring 116 needs to be increased so as to fit into the seal holding groove 114. As a result, the assembly is difficult.
In consideration of the above facts, it is an object of the present invention to provide a pretensioner for a webbing retractor, which facilitates the machining of the piston and reduces the costs, and which facilitates assembly.
According to the present invention, the pretensioner for a webbing retractor comprises: a cylinder having an inner wall, the cylinder being for receiving gas therein from a gas generating agent when the vehicle is in a state of emergency; a piston movably provided inside the cylinder for rotating the winding shaft in a direction for winding the webbing due to piston movement inside the cylinder caused by gas pressure; a sealing member formed from an elastic material in an arcuate (annular or ring-like with an angle of 360 degrees and more) shape, and provided proximate a bottom surface of the piston for sealing a gap between the piston and the inner wall of the cylinder due to the sealing member being pressed against the bottom surface of the piston by gas pressure; and a cylindrical protruding seal holding portion having an upper wall portion and an external periphery formed integrally with the bottom surface of the piston, the sealing member extending arcuately around the external periphery of the seal holding portion, and the sealing member including a side wall portion proximate the inner wall of the cylinder.
When the vehicle is in a state of emergency, the gas is supplied into the cylinder and the piston is moved by the gas pressure. When the piston is moved, the winding shaft is rotated in the direction in which the webbing is wound, the webbing is wound onto the winding shaft, and thus the slackness in the webbing is eliminated. When the piston is moved by the gas pressure, the elastic and arcuate sealing member is pressed against the bottom surface of the piston by the gas pressure such that the diameter of the sealing member is increased, and the side wall portion of the sealing member is pressed against the inner wall of the cylinder. Accordingly, the sealing member seals the gap between the piston and the inner wall of the cylinder, and the air-tightness between the piston and the cylinder is improved.
In this manner, merely by making the sealing member proximate the bottom surface of the piston, the sealing member can provide sealing between the piston and the inner wall of the cylinder, and thus, a conventional seal holding groove does not need to be formed in the piston. This facilitates the machining of the piston and reduces the costs.
Preferably, in the above-described pretensioner for a webbing retractor, a cylindrical protruding seal holding portion having an annular upper wall portion and an external periphery, is formed integrally with the bottom surface of the piston, the upper wall portion of the sealing member extending arcuately around the external periphery of the seal holding portion, and the sealing member includes a side wall portion proximate the inner wall of the cylinder.
The upper wall portion of the sealing member extends arcuately around the external periphery of the seal holding portion which is formed integrally with the bottom surface of the piston, and thus the sealing member does not come off from the piston. Therefore, the sealing member can securely provide sealing between the piston and the inner wall of the cylinder.
Further, merely by fitting the upper wall portion of the sealing member onto the external periphery of the seal holding portion which is formed integrally with the bottom surface of the piston, the sealing member can be assembled with the piston. Therefore, increasing the diameter of an O ring to fit the O ring into the seal holding groove such as in the conventional manner is unnecessary. This facilitates assembly.
More preferably, in the above-described pretensioner for a webbing retractor, the sealing member includes a side wall portion and an upper wall portion, and a discontinuity formed at the sealing member, the discontinuity being continuous with the upper wall portion and the side wall portion of the sealing member, the piston having a bottom surface side and a top surface side, wherein the discontinuity defines a path fluid communication from the bottom surface side of the piston to the top surface side of the piston.
When the pretensioner is operated, the gas flows out from the bottom surface side of the piston to the top surface side thereof through the discontinuity of the sealing member. This prevents the gas pressure inside the cylinder from excessively increasing. Gas is an example of fluid. Accordingly, a conventional vent hole does not need to be formed in the piston. This facilitates the machining of the piston and reduces the costs.
The sealing member is formed in an arcuate shape, and the discontinuity is formed at the sealing member. Thus, the diameter of the sealing member is increased more easily by the gas pressure. This can improve the sealing of the gap between the piston and the inner wall of the cylinder even more.
More preferably, in the above-described pretensioner for a webbing retractor, the gas generating agent has an ignition temperature, and the sealing member is formed of a material having a melting point no more than the ignition temperature of the gas generating agent.
The sealing member is formed of a material having a melting point no more than the ignition temperature of the gas generating agent. Thus, when the pretensioner is operated in a high temperature state such as in a fire, portions of the sealing member soften (liquefy), and the gas flows out from the bottom surface side of the piston to the top surface side thereof. This can prevent the gas pressure inside the cylinder from excessively increasing even in a high temperature state. Therefore, even if the strengths required of the cylinder, the rack, the pinion and the like are set to lower levels, the parts can be prevented from being blown off and damaged by the gas pressure.